This invention relates to a process of forming perforated ceramic fiber plates and to apparatus for carrying out the production of such plates. More particularly, the invention is directed to the production of perforated ceramic fiber plates wherein the perforations are of small diameter and are closely spaced from one another.
Perforated ceramic plates of various configurations and compositions have been disclosed in numerous patents. U.S. Pat. No. 3,954,387 to Cooper, U.S. Pat. No. 4,504,218 to Mihara et al and U.S. Pat. No. 4,673,349 to Abe et al are illustrative of the varieties of compositions and configurations of perforated ceramic plates that have been proposed. An important use of perforated ceramic plates is as burner faces of gas burners.
Domestic water heaters commonly have vertical cylindrical water tanks with diameters of at least 12 inches, most frequently in the range of about 14 to 18 inches, and metal gas burners positioned below the bottoms of the water tanks. Such water heaters represent a large potential market for perforated ceramic plates if made capable of serving as gas burner plates in lieu of conventional metal gas burners. The advantage of perforated ceramic plates for water heaters is maximized if they can function as flameless infrared burners emitting radiant energy directly to the bottoms of the upright water tanks. For practical burner use, the thickness of perforated ceramic plates should be not more than about 0.5 inch.
There are many difficult requirements imposed on perforated ceramic plates if they are to function as infrared burners under the water tanks of domestic water heaters. The metal gas burners conventionally used with water heaters have been designed for blue flame combustion at a firing rate of at least 40,000 BTU (British Thermal Units) per hour. For a perforated ceramic plate to replace the metal burner, it must be operable at a firing rate of at least 40,000 BTU per hour, preferably about 50,000 BTU per hour. Such operation makes it necessary that the perforations are not more than 0.8 inch in diameter to prevent flash back, and are so closely spaced that the perforated area is at least 25% of the burner plate area to minimize pressure drop for naturally inspirated operation with low-pressure (e.g., 4 inches water column) natural gas.
Consequently, these parameters require a burner plate with at least one square foot of its face containing the aforesaid closely spaced small perforations to permit a firing rate of at least 40,000 BTU per hour. The ceramic burner plate must also have sufficient strength to enable an unsupported plate span of at least 12 inches to resist sagging and fracturing during extended use as an infrared burner. The inclusion of ceramic fibers in such a burner plate is indispensable for strength.
A search of the technical literature reveals that the production of large, closely perforated ceramic fiber plates from an aqueous suspension of the chopped fibers has previously not even been attempted, probably because of the multiple difficult requirements such plates must meet to serve as infrared burner plates.
The aforesaid patents of Mihara et al and Abe et al mention perforated plates with dimensions ranging from approximately 3 inches by 4 inches to less than 8 inches by 8 inches. Worse yet, correspondence with the companies that own these patents has revealed that the patented products are not available because they are not being manufactured. The burner plates of the patent to Cooper are available from the patent owner but the technical data supplied by the owner shows that the largest plate offered is approximately 7.7 inches by 5.5 inches. In summary, the largest perforated ceramic fiber plate mentioned in the aforesaid patents is nearly 62 square inches but the only commercially available perforated plate is offered in the form of rectangles in sizes ranging from 9.5 square inches to 42.2 square inches.
Accordingly, a principal object of the invention is to provide a simple and rapid process of producing highly perforated ceramic fiber plates.
Another important object is to provide a commercially attractive process for vacuum-forming large, highly perforated ceramic fiber plates.
Still a further object is to enhance the economic attractiveness of such a process by eliminating the usual sintering of ceramic products.
An additional important object is to provide apparatus for the vacuum-forming of highly perforated ceramic fiber plates.
These and other features and advantages of the invention will be apparent from the description which follows.